Bava Kamma 25B

זובו של זב, ורוקו, ושכבת זרעו, ומימי רגליו, ודם הנדה ⁠– מטמאין בין במגע בין במשא.

are the ziva of a zav, and his spittle, and his semen, and his urine, and the blood of a menstruating woman, all of which impart impurity both by touching and by carrying.

ודלמא הכי נמי ⁠– לפי שאי אפשר בלא צחצוחי זיבה! אם כן, לתנייה גבי זובו; מאי שנא דקתני לה גבי רוקו? אלא משום דאתי מרוקו.

The Talmud questions this assertion: But perhaps here too the reason for the severity of ritual impurity of the semen is indeed because it is impossible for it to emerge without small drops of ziva accompanying it, but it is not due to the impurity of the semen itself. The Talmud rejects this opinion: If that would be so, let the tanna of the Mishnah teach it in the list of bodily fluids along with, meaning list it immediately following, his ziva. What is different about semen that he teaches it along with: His spittle? Rather, it is because its halakha comes from, i.e., it is derived from, that of his spittle, and therefore the semen itself imparts impurity by carrying, even if there would not be small drops of ziva accompanying it.

אמר ליה רב אחא מדפתי לרבינא: והא האי תנא לא דריש דיו, ואף על גב דלא מפריך קל וחומר! דתניא: מפץ במת מניין? ודין הוא, ומה פכין קטנים ⁠– שטהורים בזב, מטמא במת; מפץ ⁠– שמטמא בזב, אינו דין שיטמא במת?

The Talmud returns to the primary discussion concerning the principle: It is sufficient for the conclusion that emerges from an a fortiori inference to be like its source. Rav Aḥa of Difti said to Ravina: But there is this tanna who does not apply the principle: It is sufficient, at all, even though such application would not refute the entire a fortiori inference. As it is taught in a baraita: From where is it derived that a thick mat can contract ritual impurity from a corpse? It is a logical derivation based on an a fortiori inference: And just as small clay vessels, into which even a finger cannot be inserted because of their narrow opening and which remain ritually pure despite contact with a zav, nevertheless become impure if present under one roof with a corpse, then is it not right that a mat, which can become impure due to contact with a zav, will similarly become impure if present under one roof with a corpse?